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Executive Summary
Federal responsibility for oil and gas development on the U.S. outer continental shelf
(OCS) resides with the Minerals Management Service (MMS) of the U.S. Department of the
Interior (DOI). From 1954 through 198S, the last year for which statistics have been published,
OCS oil and gas development provided about 7% of total domestic oil production, about 13% of
domestic natural gas, and more than $90 billion in revenue from cash bonuses, lease rental
payments, and royalties on produced oil and gas (U.S. DOI, 1989).
The DOI's Environmental Studies Program (ESP) is the program through which MMS
conducts environmental studies on the OCS and collects information to prepare environmental
impact statements (EISs). The ESP began in 1973 under the Bureau of Land Management (BLM).
Through 1989, the ESP had invested more than $478 million in a wide variety of studies, most of
them performed by contractors. Physical oceanographic studies have amounted to approximately
22°h of the ESP budget on average, more than $7 million a year, for a total expenditure through
1988 of over $105 million for physical oceanography studies.
THE PRESENT STUDY
It appeared to MMS in 1986 that the time was ripe to assess the status of the present
program and to explore the needs for future studies. Thus, MMS requested an evaluation of the
adequacy and applicability of ESP studies, a review of the general state of knowledge in the
appropriate disciplines, and recommendations for future studies. Under the auspices of the NRC
Board on Environmental Studies and Toxicology, the Committee to Review the Outer Continental
Shelf Environmental Studies Program was formed to carry out the overall assignment. Three
panels were established, one of which, the Physical Oceanography Panel, investigated the physical
oceanographic aspects of the ESP, the subject of this report, which is the first of three in a series.
The panel based its report on several sources, including presentations from staff members
of the Environmental Studies and Environmental Modeling Branches of MMS; briefings by other,
independent scientists familiar with the work carried out in the different regions under the
support of the Environmental Studies Branch; results of a workshop on numerical modeling held
by the panel; and a review of the relevant scientific literature and documentation of MMS's
planning and implementation processes leading to various lease sales.
In reviewing the ESP's physical oceanography program, the panel evaluated the quality
and relevance of studies carried out in waters under federal control, which extend from the limits
of state jurisdictions (3-12 miles offshore) and include the central and outer continental shelf
waters and the continental slope. The panel also emphasized features and processes that control
the motion and fate of surface and near-surface oil in oceanic waters. Although the effects of oil
exploration and resource development are not constrained by political boundaries, most MMS
studies have dealt with U.S.-controlled waters and with the features, processes, and models that
are of primary importance to the movement of near-surface oil spills in these waters. This does
not imply that energy-related impacts in the coastal zone and in the lower water column and
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PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF
benthic boundary layer are unimportant-indeed, the panel recommends that MMS devote more
study to nearshore areas-but rather that the panel concentrated on questions that have driven the
major physical oceanographic study efforts of MMS. For this reason, the panel also did not
specifically consider the effects of gas blowouts.
In completing its evaluation, the panel considered four major topics:
1. The acquisition and use of physical oceanographic information by the ESP;
2. The state of knowledge of general physical oceanographic processes that are most
important for understanding and modeling the motion and fate of oil spills in the ocean;
3. The state of knowledge of the physical oceanography for each of the ESP regions,
based on all available sources; and
4. The adequacy and applicability of each of the ESP regional physical oceanography
programs, as well as the generic studies managed by the MMS headquarters office, as measured
by (a) the success of the field programs and modeling efforts in meeting ESP needs, (b)
contributions to the general state of physical oceanographic knowledge; and (c) interactions with
other agencies and the rest of the scientific community.
Chapter 1 of this report describes the history of the ESP and the structure of MMS, it also
discusses how MMS uses physical oceanographic information. Chapter 2 presents an in-depth
analysis of topics in physical oceanography that have general relevance to the ESP; it includes
information generated by MMS-funded studies as well as by non-MMS studies. Chapter 3
describes the physical oceanography and meteorology of each region and evaluates the regional
studies programs. It also evaluates work supported by the Washington Office (WO) of the ESP.
Chapter 4 summarizes the panel's conclusions and recommendations for future ESP studies.
ACQUISITION AND USE OF PHYSICAL OCEANOGRAPHIC INFORMATION BY THE ESP
Physical oceanography studies provide an important part of the basis for calculating
estimates of the transport and fate of oil spills in the ocean. These calculations are then used to
estimate potential effects of oil spills. The Oil Spill Risk Analysis (OSRA) model, developed by
the U.S. Geological Survey (USGS) in 1975, is used by MMS's Branch of Environmental Modeling
(BEM) to estimate the probability of oil spills in a specific lease area, to calculate oil-spill
trajectories from selected spill launch points (i.e., places that a spill is assumed to occur), and to
determine the probability that an environmental resource or coastline segment will be affected by
oil released from the selected launch points. Physical oceanographic and meteorological data are
required to calculate the probable trajectories of oil from spill sites and more generally to provide
background information for environmental impact assessment. Some of this information is
provided by studies funded by the ESP. Although physical oceanographic information (including
model outputs) obtained through the ESP has been used primarily as input to the OSRA model
and to prepare associated EISs, it has also been used to support biological and ecological studies
and to predict the transport of drilling muds and cuttings and other byproducts of oil exploration
and production.
The primary physical environmental information requirements of the OSRA model are
definitions of the circulation and wind fields in a study area (i.e., the output of ocean circulation
and meteorological models). BEM calculates trajectories for all OCS waters except those in the
Alaska region, where contractors calculate trajectories and provide them to BEM for input to the
OSRA model. The OSRA model then calculates the probability of oil-spill occurrence for the
selected launch sites using historical data, the number of "hits" (the number of times a spill
encounters an environmental resource target or shoreline segment), and the conditional
probabilities of impact on the resource within a previously selected time.
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EXECUTIVE SUMMARY
CONCLUSIONS
Physical Oceanographic Processes and Models of Importance to the ESP
MMS-funded studies have contributed to the dramatic increase in knowledge of the
coastal marine environment that has occurred during the past decade. These contributions have
included the development of circulation models and the observational study of circulation
patterns that may be useful for predicting oil-spill trajectories and fate. In general, MMS-
funded studies have fit in well with studies funded by other agencies (e.g., USGS, the
Department of Energy, the National Oceanic and Atmospheric Administration, and the National
Science Foundation).
MMS has relied too heavily on available circulation models. This reliance on numerical
circulation models for physical oceanographic input to the OSRA model makes it imperative that
the strengths, weaknesses, and limitations of the modeling approach be fully understood. This is
true whether the circulation models are used predictively or for spatial and temporal
extrapolation of observed data. In addition to considerations of the accuracy of the models used
attention must be paid to the time and space scales of motion that are required for accurate
trajectory simulation. The development and incorporation of oil-spill-fate models also are
important, requiring an accurate representation of the physical processes that contribute to
weathering.
Even where the circulation models used by MMS contractors are the state of the art, as is
often the case, this does not justify implicit trust in model results. Verification, intermodel
comparison, and sensitivity studies are needed. Areas for possible improvement that are common
to most numerical models are identified in Chapter 2. In all cases, it is important to test fully a
model's predictive ability against observations and to understand its behavior. Techniques to
accomplish these tasks quantitatively are becoming available but are used too seldom.
In spite of the quality of many individual physical oceanography studies, the results of
3
these modeling and t~ield studies have not been effectively integrated. The two types of
studies-modeling and field studies are usually procurred separately (either through direct MMS
procurements or through cooperation with other agencies), and this seems to have hampered their
integration by MMS. Thus, the information that has actually been used by BEM and
incorporated into environmental impact assessments in EISs has been less than the information
potentially available. The OSRA model, which is used for impact assessment by MMS, relies
extensively on the results of ocean-circulation models to estimate the circulation for a given area.
However, MMS's use of circulation-data sets based on field observations (or derived from
assimilation of field observations and model results) appears to be minimal. In other words, there
is insufficient validation and calibration of the ocean-circulation models with data derived from
field observations. Also, field data have not been sufficiently used as independent sources of
data to describe the circulation for input to the OSRA model. This problem also was noted in
reviewing the regional programs; in general, physical oceanographic field studies have been
extensive, but the resulting data have been underutilized. BEM has recently begun to institute
changes that the panel believes will improve its use of OSRA (pers. comm., MMS, 1990~.
The lack of integration of field-derived information into the models and the impact
assessments in EISs contrasts with the summary descriptions of regional ocean circulation in the
EISs, where field-derived information is used more extensively than in the impact assessments.
Field data from actual spills-particularly large ones, such as the recent spill in Prince William
Sound would also be useful in this regard. A related problem is that the best available
circulation models are not always used for the OSRA calculations: this was notable in some OSRA
, .
calculations done for the Atlantic region (including the OSRA calculation for the EIS for lease
sale 96~.
Another difficulty with the OSRA models is that the wind fields used in calculating spill
trajectories for the Atlantic, Pacific, and Gulf of Mexico regions have not been the same as the
wind fields used to drive the circulation models. Transition-probability matrices (i.e., matrices
giving the probabilities that a variable will change from each possible state to every other possible
state) have been used to generate wind fields for the OSRA model. The matrices, until recently,
have been based on observations at a limited number of stations; since 1989, the transition
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PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF
probability matrices have been abandoned in favor of wind fields based on meteorological
models. The use of only a few stations could have led to inaccuracies in the resultant trajectory
calculations, because the true spatial variability of winds is not adequately represented. The
potential for such inaccuracies is highlighted by recent detailed analyses of winds observed
onshore and offshore, which found large spatial and temporal variability in the structure of the
wind field and in the coherence between onshore and offshore stations. This variability is
greatest near the coast, where land-sea interaction is an important factor and where spilled oil is
more likely to have long-lasting impacts.
The panel identified several processes that must be better understood and included in
models that estimate oil-spill motion at or near the surface over the time frame of interest.
In most cases, oil spills in the ocean result in surface slicks that drift, spread, and weather in
response to environmental conditions. Petroleum spills in the marine environment undergo
relatively rapid weathering. Evaporative losses, dispersion, and dissolution into the water column
all occur within a few days to a week after a spill (NRC, 1985~. After 30 days, most surface
slicks are well weathered (Mackay et al., 1983; Koons, 1987; NRC, l989b). Thus, the physical
oceanographic processes that are most important to the ESP from the perspective of the fate and
the effects of spilled oil (and the immediate containment and cleanup of spilled oil) are those that
most influence the motion of oil at or near the surface within about 30 days of an oil spill. Oil
spills might occur at any time during the life of a lease, and so knowledge of the inherent
variability of the physical oceanography over seasonal and interannual time scales also is
important. Although physical oceanography does not usually consider the behavior of oil and
other contaminants per se, oil-spill-fate modeling is important for accurate prediction of oil-spill
behavior. This report emphasizes the spreading and dispersion of oil, because they are the most
closely tier! to near-surface physical oceanographic processes. Spreading is one of the most
important processes in oil-spill dynamics, because it determines the areal extent of spilled oil and
affects the various weathering processes influenced by surface area. Dispersion is generally
assumed to result from wind-generated breaking waves dispersing oil in the water column. Both
processes are poorly understood, but both depend critically on the interactions of the wind, the
surface-wave field, the response of near-surface waters, and the use of chemical dispersants.
The presence and dynamics of ice clearly are important for modeling oil trajectories in
most Alaskan waters (and some nearshore New England waters during especially cold winters);
ice conditions are influential in determining the movement and final disposition of spilled oil. In
addition, interactions between oil and sea ice are poorly understood. These interactions are
extremely complex, depending on the percentage of ice cover, ice motion, temperature, wind,
duration of ice cover, and the history and location of ice-oil contact. Few models account for
oil-ice interaction.
Regional Oceanography
The U.S. continental margins are divided into four regional jurisdictions covered by the
regional offices of ESP: the Alaskan coast, the Pacific coast, the Gulf of Mexico, and the
Atlantic coast. In addition, some studies that apply to the entire OCS are funded by the WO. The
four regions are distinguished by more than MMS's internal division of responsibility; the regions
have fundamental differences in geology, topography, and bathymetry and in the processes that
control the circulation of shelf waters and affect the motion of oil in surface waters of the
regions. These differences also are apparent between subregions within each region. In general,
the physical oceanography of all of the major continental margins is reasonably well known,
especially from a basinwide, descriptive point of view.
Evaluation of Regional Programs and Washington Office Programs
In contrast to the regional offices' goals of data collection, analysis, and synthesis, the
efforts of the WO have focused on supporting regional studies, addressing generic process and
modeling issues, and summarizing or documenting previous studies. The number of physical
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EXECUTIVE SUMMARY
s
oceanographic studies funded by the WO has been limited, but according to the material
available, studies completed by the WO have addressed important areas and have been completed
with timely products of generally good quality. Given the importance of generic studies to the
overall ESP effort, the WO is small compared to its regional counterparts.
Several important generic research efforts now carried out by the regional offices instead
seem appropriate for the WO under the generic studies program. The mandate to complete these
overview or generic efforts clearly belongs with the WO, and the management structure of MMS
should reflect that.
In evaluating the physical oceanographic components of the ESP in the four regional
offices and the WO, the panel noted that although regional efforts vary, too little of the work
carried out for MMS has been published in the open, refereed literature. This is particularly true
of the modeling and model-data intercomparison studies. Publication in the open literature could
improve the quality of MMS-funded efforts through the peer-review process and substantially
increase, at little cost, the body of knowledge available to the oceanographic community. MMS's
recent efforts in this direction are commendable.
RECOMMENDATIONS
The panel makes three general recommendations for future ESP physical oceanography
and oil-spill studies, and for the use of results of these studies by BEM and in EISs. Each
general recommendation has several associated specific recommendations. The recommendations
(see Chapter 4) are briefly summarized below.
1. The Minerals Management Service should! support continuing studies on relevant
physical oceanographic and meteorological processes and features that are poorly understood,
poorly parameterized in existing models, or poorly represented by existing modeling
methodology. Improvements should continue to be incorporated into the OSRA model.
a. MMS should support continuing investigations of surface-layer physics, aimed at
Improving basic understanding and modeling.
b. MMS should continue to support studies that lead to understanding and modeling
of oil-spill-fate processes.
c. MMS should support additional observational and modeling studies of sea-ice.
d. MMS's recent moves to adopt consistent methods in calculating oil-spill trajectories
at the sea surface and calculating the underlying currents are commended, they should continue.
e. MMS should continue to improve the meteorological input to oil-spill-trajectory
simulations to account correctly for the spatial and temporal structure of the wind field.
f. MMS should give more consideration to extreme events (e.g., hurricanes) that might
lead to higher spill probability and more rapid water and oil motion.
g. In future trajectory simulations, MMS should incorporate a methodology to address
the inherent variability in the wind field and the current field at small (i.e., subgrid) space and
time scales.
h. MMS's use of drifters to represent oil spills should be extended to actual field trials
in varied areas of the OCS, and in similar regions worldwide as the opportunities occur.
2. The Minerals Management Service should reduce its present overreliance on model
results until the models can be more fully tested and verified; such testing will require sensitivity
analyses and model intercomparisons. Instead, MMS should use its extensive field observational
clata base more fully. Verification will require close cooperation between field scientists and
modelers.
a. MMS should seek a more balanced integration of model and field-data products for
future trajectory calculations.
b. For scientific credibility, it is imperative that MMS carry out detailed sensitivity
studies for all modeling work.
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aspects of the ESP.
6
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PHYSICAL OCEANOGlRAPHY OF THE U.S. OUTER CONTINENTAL SHELF
c. It is also imperative for scientific credibility that MMS perform systematic model
ntercomparison and verification against field data.
d. MMS should require closer cooperation between field scientists and modelers in
future MMS-sponsored physical oceanographic field programs. More input from field scientists
is also needed in model design, application, and verification.
e. Given the present level of understanding for most shelf regions of interest to the
OCS leasing program, a carefully integrated program using field observations and numerical
hydrodynamic modeling is suggested to provide a description of the circulation needed for input
to the OSRA model.
f. MMS should strengthen its ability to seize the scientific study opportunities
provided by accidental oil spills, such as the recent Exxon Valdez spill in Prince William Sound.
The Exxon Valdez spill also illustrates the importance of analyzing worst-case scenarios.
g. Although technically the ESP's jurisdiction covers the OCS, MMS should consider
oil spills occurring shoreward of the OCS. The panel notes that OCS oil can be spilled inshore of
the OCS.
3. Program priorities and operating procedures in the ESP should be modified as
necessary to ensure that improved scientific input Is obtained at all stages of ESP operation in all
regions, that available data from cooperating agencies are used, that development of a better-
integrated national program continues, and that study result are published more often in the
peer-reviewed scientific literature.
a. MMS needs a more appropriate balance between national and regional priorities.
b. MMS should obtain more external scientific input and use it more fully in many
c. MMS's cooperation with other agencies is commended; it should continue.
d. MMS should continue its efforts to develop a better-integrated national program
while maintaining the regional office structure.
e. MMS should continue to strengthen its program to have the results of its studies
presented at scientific meetings and published in the open, peer-reviewed scientific literature.
Representative terms from entire chapter:
physical oceanography
